Motor control system



June 28, 1960 D. w. FATH MOTOR CONTROL SYSTEM 2 Sheets-Sheet 1 FiledMarch 10; 1958 21$ 4 O NOQM. OAC

June 28, 1960 D. w. FATH 2,943,250

MOTOR CONTROL SYSTEM Filed March 10, 1958 2 Sheets-Sheet 2 f y. D Q. a a0P 0 INPUT J '1 '5 NLI aa M 3 L461 Ac PBS Bl 0-/OP| I 4\/ /OPZ o mPu'r\L K? D k 4 D 0 Ag" P cl.

2 T 1/ arm 0 INPUT United States Patent MOTOR CONTROL SYSTEM Douglas W.Fath, Brookfield, Wis., assignor to Cutler- Hammer, Inc., Milwaukee,Wis., a corporation of Delaware Filed Mar. 10, 1958, Ser. No. 720,401

8 Claims. (Cl. 318-143) This invention relates to motor control systemsand more particularly to voltage regulator networks for adjustablevoltage motor-generator systems.

While not limited thereto, the invention is especially applicable tovoltage regulator networks of the magnetic amplifier type having currentlimit acceleration and deceleration control of a direct current motoremployed for operating a skip hoist or the like.

In adjustable voltage systems for controlling motor acceleration inpredetermined steps, current limit control is normally provided toprevent excessive current flow in the motor-generator loop during theacceleration periods following the incremental changes in generatorfield excitation. To this end, a feedback signal proportional togenerator armature current is provided to limit the rate of change ofgenerator field excitation. Due to delays in the system networks and inresponse of the generator to such current limit control, the generatorarmature voltage increases rapidly to each incremental value. Thiscauses the motor current to overshoot to excessive values creatingundesirable conditions dangerous to apparatus during transitory periodsbetween initiation of current limit control and response of thegenerator armature voltage thereto.

It is therefore a general object of the invention to provide improvedmeans to overcome the aforementioned undesirable conditions.

A more specific object of the invention is to provide improved currentlimit control means for an adjustable voltage system.

A still more specific object of the invention is to provide such currentlimit control means with improved current overshoot limit control meanseffective between initiation of current limit control and response of acontrollable device thereto.

Another specific object of the invention is to provide an improvedvoltage regulator network of the current limit acceleration type for anadjustable voltage system having means to precipitate current limitcontrol.

Another specific object of the invention is to provide a magneticamplifier type current limiting volt-age regulator having improvedresponse whereby to anticipate and initiate current limit control.

Other objects and advantages of the invention will hereinafter appear.

While the apparatus hereinafter described is effectively adapted tofulfill the objects stated, it is to be understood that I do not intendto confine my invention to the particular preferred embodiment ofcontrol system disclosed, inasmuch as it is susceptible of variousmodifications without departing from the scope of the appended claims. I

In the drawings,

Figure l diagrammatically illustrates a control system constructed inaccordance with the invention; and

Figs. 2, 3 and 4 graphically depict operating characteristics of certaincomponents of the system of Fig. 1.

In 'Fig. 1 the relay contacts are'related to the relay ice winding by 1and 2 sufiixed to the relay reference char-,

acter and the numeral within the winding indicates the number ofcontacts.

Referring to Fig. 1, there is shown a motor-generator regulator andtesting control system wherein a generator mechanically driven by aprime mover PM supplies power to an electrical motor, the latter beingemployed to drive a skip hoist or the like for charging a blast furnace. The motor is provided with an armature MA and a. shunt fieldwinding MP, the latter being energizable from a suitable direct currentpower supply source.

Armature MA is connected in a loop circuit with generator armature GA,normally open run relay contacts RRl and series armature resistor R1. Avoltmeter V is connected across generator armature GA for reasonshereinafter described. Shunt field winding GF of the generator issupplied with power from power supply lines AC1 and AC2 through apre-amplifier PRA and a generator field power amplifier GFPA, lines AC1and AC2 being connected to a suitable alternating current power supplysource (not shown). An accelerating current limit amplifier ACLA isprovided for deriving a signal from series armature resistor R1 and forcontrolling power amplifier GFPA during motor acceleration. Theaforementioned amplifiers are illustrated as being of the magnetic type.

In actual practice, motor armature MA is normally provided withreversing circuits and field winding MP is provided with controlresistor circuits. Also a decelerating current limit amplifier isnormally provided to limit generator-motor loop current duringdeceleration. These elements have not been shown to avoid complicatingthe drawings.

Pre-amplifier PRA is provided with a pair of power windings P, a biaswinding B, a feedback winding PB and an input signal winding IS. Powerwindings P are connected in a well known manner for alternate half-waveenergization through respectively associated half-wave rectifiers RT1and RT2 across power supply lines AC1 and AC2, rectifiers RT1 and RT2rendering the power windings self-saturating by blocking thedesaturating half cycle from each power winding. The energizing circuitsof the power windings also are provided with a rectifier bridge RB1having its input terminals connected to line AC1 and the junction ofrectifiers RT1 and RT2, the junction of power windings P being connectedto line AC2. The positive and negative output terminals of bridge RB1,these being the output terminals of the preamplifier, are connectedthrough a resistor R2 across signal winding S of power amplifier GFPA.

Bias winding B of pre-amplifier PRA is connected through a biasadjusting resistor R3 across the positive and negative output terminalsof a rectifier bridge RBZ having its input terminals connected tosecondary winding S1 of a constant voltage transformer CVT, the latterhaving its primary winding P connected for energization across lines AC1and AC2. Feedback winding PE is connected through resistor R4 acrossgenerator armature GA to provide the pre-amplifier with a feedbacksignal proportional to generator armature voltage. Input signal windingIS is connected in series with rheostats RHl, RH2 and RH3 and normallyopen run relay contacts RR2, in that order, across the aforementionednegative and positive output terminals of rectifier bridge RB2. Normallyopen reference voltage contacts RV2 and RVl are connected acrossrheostats RH2 and RH3, respectively, for shunting the latter. I

A reference voltage test network comprising a reference test selectorswitch RTS and a speed selector switch SS is provided for testing theoutput of the regulator without actually running the skip hoist motorthereby to facilitate adjustment of the input signal voltages to thepre-amplifier Patented June 28, 1960 for a plurality of predeterminedmotor speeds. Speed selector switch SS may be of the manual type or thelike having a movable contact arm and 18, 2S and 6S designatingoperating position stationary contacts, the latter being indicative ofthree predetermined skip hoist speeds. The 68 designating contact isconnected to the junction of rheostats RH1 and RH2, the 2S designatingcontact is connected to the junction of rheostats RH2 and RH3, and the1S designating contact is connected to the junction of rheostat RH3 andcontacts RR2. The reference test selector switch RTS is provided with apair of contact sets RTSI and RTSZ, each having a movable contact armmechanically connected to one another, an off position and test andnormal designating operating position contacts. The test contact ofcontact set RT S1 is connected to the movable contact arm of switch S5while the movable contact arm of contact set RTSl is connected to thepositive output terminal of bridge RB2.

The movable contact arm of contact set RTS2 is connected through theoperating coil of a power relay OAC to the positive output terminal ofbridge RB2, while the test contact thereof is connected directly and thenormal contact thereof is connected through normally open contacts UV2of undervoltage relay UV to the negative output terminal of bridge RB2.The operating coil of relay UV is connected through a normally openreset button switch RB and a normally closed stop button switch SB forenergization across a suitable direct current source, normally opencontacts UV1 being connected in shunt of switch RB for maintainingenergization of the undervoltage relay. The UV relay circuit is normallyprovided with protective devices as indicated by the broken conductor.

Generator field power amplifier GFPA is provided with a pair of powerwindings P, a current limit winding CL, a bias winding B and theaforementioned signal winding S. Power windings P are connected foralternate halfwave energization through respectively associated halfwaverectifiers RT3 and RT4 across conductors 2 and 4; the energizingcircuits of the power windings also including a rectifier bridge RB3having its input terminals connected to conductor 2 and the junction ofrectifiers RTS and RT4, and the junction of windings P being connectedto conductor 4. Conductors 2 and 4 are connected through normally opencontacts OAC1 and OAC2, respectively, to lines AC1 and AC2. The positiveand negative output terminals of rectifier bridge RH3, these being theoutput terminals of the power amplifier, are connected across generatorshunt field winding GF.

Current limit winding CL is connected across the output terminals ofaccelerating current limit amplifier ACLA. Bias winding B of the poweramplifier is connected through a bias adjusting rheostat RH4 to thenegative and positive output terminals of a rectifier bridge RB4 havingits input terminals connected to secondary winding S2 of theaforementioned constant voltage transformer CVT. While two rectifierbridges RB2 and RB4 have been shown for ease of illustration, it will beapparent that a single bridge could be employed in place thereof. Theaforementioned signal winding S of the power amplifier is connected tothe output terminals of pre-amplifier PRA to connect the amplifiers inseries relation.

Accelerating current limit amplifier ACLA is provided with a pair ofpower windings P, a feedback signal winding FBS, a bias winding B and anovershoot limit winding OSL. Power windings P are connected foralternate half-wave energization through respectively associatedhalf-wave rectifiers RTS and RT6 across power supply lines AC1 and AC2in parallel with pre-amplifier PRA. The energizing circuits of the powerwindings are also provided with a rectifier bridge RBS having its inputterminals connected to line AC1 and the junction of rectifiers RTS andRT6, the junction of power windings P being connected to line AC2. Thepositive and negative output terminals of rectifier bridge RBS, thesebeing the output terminals of the current limit amplifier, are connectedto the aforementioned current limit winding CL of power amplifier GFPA.

Feedback signal winding FBS is connected across the aforementionedseries armature resistor R1 in the motorgenerator loop circuit. Biaswinding B of amplifier ACLA is connected through a bias adjustingrheostat RHS to the negative and positive output terminals of rectifierbridge RB4. Overshoot limit winding OSL is connected to the positiveoutput terminal of pre-amplifier PRA and through a capacitor C1 andresistors RS and R6 of a filter network F in series to the negativeoutput terminal of preamplifier PRA. Filter F is also provided with acapacitor C2 connected in shunt of resistor R5, capacitor C1 andover-shoot limit winding OSL.

The operation of the regulator and testing control system of Fig. 1 willnow be described with reference to the characteristic curves ofpre-amplifier PRA, accelerating current limit amplifier ACLA andgenerator field power amplifier GFPA shown in Figs. 2, 3 and 4,respectively.

, Let it be assumed that suitable alternating current power is appliedto supply lines AC1 and AC2 and that direct current power is connectedto the positive and negative terminals of the undervoltage relay circuitin the lower portion of Fig. 1. As a result, an energizing circuit isestablished through primary winding P and secondary winding S1 oftransformer CVT, rectifier bridge RB2 and resistor R3 to bias winding Bof pre-amplifier PRA thereby to bias the amplifier to cut-off as shownby arrowB in Fig. 2. Therefore, although power windings P of thepre-amplifier are connected to lines AC1 and AC2, cur

' rent does not flow therein. And energizing circuit is also establishedthrough secondary winding S2 of transformer CVT, rectifier bridge RB4and rheostat RH4 to bias Winding B of the power amplifier thereby tobias the latter to cut-oif as shown by arrow B in Fig. 4. A furtherenergizing circuit is established from the output terminals of rectifierbridge RB4 through rheostat RHS to bias winding B of acceleratingcurrent limit amplifier ACLA thereby to bias the latter beyond cut-offas shown by arrow B in Fig. 3. Let it further be amumed that referencetest selector switch RTS is turned to its NORM designating operatingposition for normal operation of the regulator.

Undervoltage relay UV is energized by momentarily pressing reset buttonRB to complete a circuit for the operating coil thereof through stopswitch SB. Contacts UV1 complete a holding circuit therefor in shunt ofswitch RB and contacts UV2 complete an energizing circuit from thepositive output terminal of rectifier bridge RB2 through the operatingcoil of power relay OAC, contact set RTS2 and contacts UV2 to thenegative output terminal of bridge RB2. Relay OAC energizes and closescontacts OAC1 and OAC2 to connect power windings P of power amplifierGFPA through conductors 2 and 4 to supply lines AC1 and AC2.

To start the motor, run relay contacts RR1 and RR2 are closed. ContactsRR1 connect motor armature MA through resistor R1 across generatorarmature GA to complete the motor-generator loop circuit. Motor shuntfield winding MP is separately excited from a suitable direct currentsource through field weakening resistors or the like (not shown) sothat, when the generator supplies a voltage to the motor as hereinafterdescribed, the latter starts. Contacts RR2 complete an energizingcircuit from rectifier bridge RB2 to input signal winding IS throughrheostats RHl, RH2 and RH3 in series connection thereby to bias thepre-amplifier ,on as indicated by arrow IS in Fig. 2.

It will be apparent that the output of pre-amplifier PRA is applied topower amplifier GFPA and that the output of the latter is applied toexcite generator field winding GE. As is well known, the generatoroutput voltage is a function of the field flux and the speed and in themotor-generator system the motor speed is a function of the generatoroutput voltage. Therefore, the motor speed is controllable by themagnitude of the preamplifier input signal. To this end, the resistancevalues of rheostats RH1, RHZ and RH3 have been predetermined andadjusted by the movable arms thereof to afford a first increment ofmotor speed 18 when all three rheostats are in series connection withinput signal windings IS, to afiord a second increment of motor speed 28when rheostat RH3 is shunted by contacts RV1, and to afford a thirdincrement of motor speed 65 when rheostat RHZ also is shunted bycontacts RV2 so that rheostat RH1 alone remains in the input signalwinding circuit.

As hereinbefore described, energization of input signal winding ISbiases the pre-amplifier on. As a result, the pre-amplifier afiords anamplified output signal which is applied to signal winding S of poweramplifier GFPA thereby to bias the latter on as indicated by the arrow Sin Fig. 4. The power amplifier in turn affords an amplified outputsignal to excite generator field winding GF. The generator being drivenby a prime mover, the generated output voltage is applied through theloop circuit to start the motor. A signal proportional to the generatorarmature output voltage is also fed back through resistor R4 to feedbackwinding FB of the pro-amplifier to oppose input signal winding IS asshown by arrow PE in Fig. 2. Thus, the generator output voltage isregulated in the first instance by balancing reference input signalampere turns developed in winding IS against generator armature voltagefeedback ampere turns developed in winding FB. An increase in referenceinput signal increases the generator field current and in turn thegenerator armature voltage. The increase in generator armature voltageis fed back to decrease the output from the pre-amplifier to bias thelatter to operating point OP as shown in Fig. 2.

As hereinbefore described, energization of bias winding B of currentlimit amplifier ACLA biases the latter a predetermined amount beyondcut-ofi. The current limit amplifier regulates generator armaturecurrent during acceleration by balancing bias ampere turns developed inWinding B thereof against feedback signal ampere turns developed inwinding FBS, the latter ampere turns being proportional to the armaturecurrent. To this end, a feedback signal is derived across resistor R1 inthe motorgenerator loop and applied to feedback signal winding FBS tooppose bias winding B as shown by arrow PBS in Fig. 3. Winding B biasesthe current limit amplifier well beyond cut-01f so that the latterremains in the cut-off condition when normal load armature current isbeing drawn from the generator as shown by arrow NLI in Fig. 3. Theenergization of Winding B is preferably adjusted by rheostat RHS so thatapproximately 140 percent generator current ju-st starts to turn thecurrent limit amplifier on as shown by arrow 1.4GI in Fig. 3, thereby tomaintain armature current during acceleration at a safe value ashereinafter described.

The output of current limit amplifier ACLA is applied to theaforementioned current limit winding CL of power amplifier GFPA in adirection to oppose the pre-amplifier output to signal winding S tolimit the generator armature current during acceleration to a safevalue. When the motor, and the skip hoist driven thereby, is startedfrom rest, an input signal voltage calling for a predetermined voltagefrom the generator is applied to pre-amplifier PRA. Without the currentlimit amplifier, the generator voltage would increase rapidly to thepredetermined value, causing the motor to draw excessive armaturecurrent. With the current limit amplifier in circuit, as soon as thegenerator voltage increases to a value which causes excessive generatorarmature current to flow, feedback signal winding FBS turns the currentlimit amplifier on to operating point 0P1 as shown by arrow PBS in Fig.3. The resultant amplified output which is proportional to the excessarmature current is applied to current limit winding CL of the poweramplifier to oppose the pre-amplifier output thereto as shown by arrowCL in Fig. 3. As a result, the output of the power amplifier isdecreased to operating point OP thereby to decrease the generatorarmature voltage. In turn, the feedback signal is decreased to a valuerepresented approximately by arrow 1.4GI, thereby to decrease thecurrent limit amplifier output to a value represented by operating point0P2. Thus, the current limit amplifier allows the generator voltage torise just fast enough to maintain the armature current at a safe valueduring the acceleration period.

The motor is accelerated to a second incremental speed by closingcontacts RV1 to shunt rheostat RH3 and increase the input signal voltageto winding IS a corresponding amount. In like manner, the motor isaccelerated to a third incremental speed by closing contacts RV2. Afterthe motor has accelerated to each of the aforementioned incrementalspeeds, the armature cur rent decreases to the normal running valuerepresented by arrow NLI in Fig. 3 and the current limit amplifier isturned off.

Due to delays in the circuits between the time that the armature currentincreases to an excessive value and the time that the current limitcontrol comes into play to decrease the armature voltage, the armaturevoltage tends to overshoot thereby permitting the motor to draw currentin excess of a safe value. To prevent this dangerous transitorycondition, amplifier ACLA is provided with overshoot limit winding OSLpreferably energized from the output terminals of the preamplifier, itbeing apparent that winding OSL could as well be energized from othersuitable points in the regulator. Network F nlters the pre-amplifieroutput voltage which is then applied through capacitor C1 to windingOSL. Thus, when the rectified input signal voltage wave turns thepre-amplifier on, a current pulse proportional to the initial rate ofchange of pro-amplifier output and having a steep wave front from whenceit decays in approximately one second is applied to the overshoot limitwinding as shown by arrow OSL in Fig. 3. This pulse initiates operationof the current limit amplifier to prevent the armature voltage fromovershooting as aforementioned.

To test the output of the voltage regulator without I lector switch RTSis turned to its TEST designating 0pcrating position. Relay OAC ismaintained energized through contact set RTS2 to connect power winding Pi of amplifier GFPA to lines AC1 and AC2.

for exemplary purposes, it may be assumed that the aforementioned threeincremental steps of motor speed.

13, 2S and 6S require predetermined values of 100, 20 and 300 armatureoutput volts, respectively. With test selector switch SS in its 68designating operating position as shown, rheostat RH1 is adjusted sothat voltmeter V indicates a generator armature voltage of 300 volts.Next, selector switch SS is turned to its 28 designating operatingposition and rheostat RH2 is adjusted so that voltmeter V indicates agenerator armature voltage of 200 volts. Finally, selector switch SS isturned to ts 15 designating operating position and rheostat RH3 is adusted so that voltmeter V indicates a generator armature voltage ofvolts. It will be apparent that seting of selector switch SS inoperating positions 25 an closure of contacts RV1 and RV2, respectively,under normal operating conditions. The regulator is reset for normaloperation by turning selector switch RTS to its NORM designatingoperating position and resetting the undervoltage relay by pressingswitch RE.

The aforementioned testing control system is disclosed 6S shuntsrheostats RH3 and RH2 to simulate the 7 and claimed in Douglas W. Fathcopending application Serial No, 720,397, filed March 10, 1958.

1. In; an adjustable voltage motor-generator control system providedwith a voltage regulator responsive to predetermined incrementaltvaluesof input signal for. controlling generator field, excitation inrespectively associated steps thereby to control the motor speed, andcurrent limit control: means: for controlling the generator armaturevoltage as a. function of the generator armature current whenever thelatter exceeds a predetermined value, eachv increase in input: signalvcausing: the generator armature voltage to increase rapidly to theassociated incremental value thereby permitting the motor to drawexcessive current from thegenerator airmature during a time interval.before the current limit control means normally acts to control thegenerator armature voltage, the improvement comprising means responsiveto a change in. the input signal for providing a current overshoot,limit. control. signal proportional to such change, and means responsiveto said. current overshoot limit control signalfor rendering saidcurrent limit control means operative to control the generator armaturevoltage during said time interval.

2. In an adjustable voltage motor-generator control system, thecombination with. a voltage regulator comprising magnetic amplifiermeans responsive to predetermined incremental. valuesof input signal forcontrolling generator field energization in corresponding steps therebyto accelerate the. motor, and current limit control means comprising amagnetic amplifier responsive to the motor-generator loop currentexceeding a predetermined value for controlling said amplifier meansthereby to control the generator field energization and armature voltageto limit the. loop current to said predetermined value, an incrementalchange in input signal causing the generator armature voltage to riserapidly to the corresponding value thereby. permitting the motor to drawexcessive current from the generator armature for a transientperiodbefore the current limit amplifier normally responds to controlthe generator armature voltage, of means for deriving a currentovershootlimit control signal proportional to a change in the input signal fromsaid amplifier means before the motor current reaches an excessivevalue, and a current overshoot limit winding responsive to saidovershoot signal forprecipitating the response ofsaid current limitamplifier.

3. In an adjustable voltage motor-generator control system wherein thearmature of the motorand generator are connected in a loopcircuit andthe generator field energization is controlled to adjust the speed ofthe motor,

in combination, a source of input signal adjustable in steps, apre-amplifier responsive to said input signal for providing an outputsignal which varies in steps proportional to the steps of said inputsignal, a, power amplifier responsive to the output signal of saidpreamplifier for providing a correspondingly adjustable output signalfor energizing the field winding of the generator, a current limitamplifier responsive to the current in the loop circuit for providing anoutput signal for controlling said power amplifier to adjust thegenerator field windingenergization in proportion to and Whenever thecurrent in the loop circuit exceeds a predetermined value, each step ofadjustment of said input signal causing the generator armature voltageto change rapidly to the new value thereby permitting the motor currentto become excessive during a time interval before said current limitamplifier and said power amplifier normally function to control thechange in generator armature voltage, means responsive to said inputsignal for providing a current over-shoot limit control signal which isa function of the rate of change of said input signal, and meansresponsive to said current overshoot limit control signal' for causingsaid current limit amplifier to provide an output signal for controllingsaid power amplifier to regulate the rate of change of generator fieldenergization during said time interval.

4. The invention defined in claim 3', wherein said current limitamplifier is a magnetic amplifier provided with an overshoot limitwinding for controlling the output thereof during said time interval,and the output signal of said pre-amplifier is connected to saidovershoot limit winding.

5.. The invention defined in claim 3, wherein said current: limitamplifier is a magnetic amplifier having an overshoot limit winding forcontrolling the outputthereof during said time interval, and said meansfor providing a current overshoot limit control signal comprises acapacitor connecting the output signal of said pre-amplifier to saidovershoot limit' winding.

6. In an adjustable voltage motor-generator control system wherein thearmatures of the motor and generator are connected in a loop circuit andthe generator field energization is controlled to adjust the speed ofthe motor, in combination, an. alternating current source, meansconnected to said source for providing an input signal adjustable insteps, a magnetic preamplifier connected to said source and responsiveto said input signal for providing a control. signal" which varies insteps in accordance with the steps of said input signal, a magneticpower amplifier connected to said source and responsive to said controlsignal for providing a correspondingly adjustable output current forenergizing. the field winding of the generatona current-limit magneticamplifier connected to said source and responsive to the current in theloop circuit for providing an output signal for controlling said poweramplifier to adjust the generator field Winding energization wheneverthe current in the loop circuit exceeds a predetermined adjustablevalue, each step of ad justment of said input signal causing thegenerator armature voltage to change rapidly to a new value thereby.permitting the motor current to become excessive during a time intervalbefore said current limit-amplifier and said power amplifier normallyfunction to control the rate of change in generator armature voltage,means responsive to each step of change. insaid input signal forproviding a current overshoot limit control signal pulse, and a controlwinding in said current limit amplifier responsive to said pulse forcausing the latter to provide an output signal for controlling saidpower amplifier in a direction to regulate the rate of change ofgenerator field energization during said time interval.

7. The invention defined in claim 6, wherein said means for providingsaid pulse comprises a capacitor connected between a point in the inputcircuit of the system and said control Winding.

8. The invention defined in claim 7, wherein said pulse is proportionalto the initial rate of change of current in said input circuit and has asteep wave front from whence it decays in a predetermined time interval,together with a filter in series with said capacitor;

References Cited in the file of this patent UNITED STATES PATENTS2,743,151 Hunt et a1. Apr. 24, 1956 2,848,673 Pettit et al Aug. 19, 19582,853,668 Moore Sept. 23, 1958'

